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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.reference.basic_waitable_timer.cancel_one"></a><a class="link" href="cancel_one.html" title="basic_waitable_timer::cancel_one">basic_waitable_timer::cancel_one</a>
</h4></div></div></div>
<p>
          <a class="indexterm" name="boost_asio.indexterm.basic_waitable_timer.cancel_one"></a> 
Cancels
          one asynchronous operation that is waiting on the timer.
        </p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">cancel_one</span><span class="special">();</span>
</pre>
<p>
          This function forces the completion of one pending asynchronous wait operation
          against the timer. Handlers are cancelled in FIFO order. The handler for
          the cancelled operation will be invoked with the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error</span><span class="special">::</span><span class="identifier">operation_aborted</span></code>
          error code.
        </p>
<p>
          Cancelling the timer does not change the expiry time.
        </p>
<h6>
<a name="boost_asio.reference.basic_waitable_timer.cancel_one.h0"></a>
          <span class="phrase"><a name="boost_asio.reference.basic_waitable_timer.cancel_one.return_value"></a></span><a class="link" href="cancel_one.html#boost_asio.reference.basic_waitable_timer.cancel_one.return_value">Return
          Value</a>
        </h6>
<p>
          The number of asynchronous operations that were cancelled. That is, either
          0 or 1.
        </p>
<h6>
<a name="boost_asio.reference.basic_waitable_timer.cancel_one.h1"></a>
          <span class="phrase"><a name="boost_asio.reference.basic_waitable_timer.cancel_one.exceptions"></a></span><a class="link" href="cancel_one.html#boost_asio.reference.basic_waitable_timer.cancel_one.exceptions">Exceptions</a>
        </h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">boost::system::system_error</span></dt>
<dd><p>
                Thrown on failure.
              </p></dd>
</dl>
</div>
<h6>
<a name="boost_asio.reference.basic_waitable_timer.cancel_one.h2"></a>
          <span class="phrase"><a name="boost_asio.reference.basic_waitable_timer.cancel_one.remarks"></a></span><a class="link" href="cancel_one.html#boost_asio.reference.basic_waitable_timer.cancel_one.remarks">Remarks</a>
        </h6>
<p>
          If the timer has already expired when <code class="computeroutput"><span class="identifier">cancel_one</span><span class="special">()</span></code> is called, then the handlers for asynchronous
          wait operations will:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              have already been invoked; or
            </li>
<li class="listitem">
              have been queued for invocation in the near future.
            </li>
</ul></div>
<p>
          These handlers can no longer be cancelled, and therefore are passed an
          error code that indicates the successful completion of the wait operation.
        </p>
</div>
<div class="copyright-footer">Copyright © 2003-2025 Christopher M.
      Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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